is_lock_free atomic_bool

Question

Hi

Came to know that atomic_bool is not lock free and hence we have spinlock. Refer code below (https://www.sololearn.com/en/compiler-playground/ciuT8ghwcf38):
#include <iostream>
#include <atomic>
using namespace std;

int main()
{

atomic<int> a(2);
cout << a.is_lock_free() << endl;

atomic<bool> b(false);
cout << b.is_lock_free() << endl;

return 0;
}

It shows b variable as lock free. Still spin lock is needed?

Answer 1

std::atomic<bool>::is_lock_free() == true means atomic reads/writes are fast and don’t use a mutex.
But atomic<bool> only protects itself, not a block of code.
Spinlocks are still needed when you want to protect a critical section, not just a variable.

So yes even if atomic<bool> is lock-free, spinlocks still have their use

Answer 2

Do you mean atomic<bool> b;

If(b)//b is atomic and works with threads fine
{
//critical section and not thread safe
}

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If above is true (I belive it is) , then mutex is needed always. Right ? How spin lock is helping there without mutex ?